1. Field of the Invention
The present invention relates to an X-ray CT apparatus having multi-column detectors and a method of creating correction data for X-ray CT, and more particularly, to a technology that is used when creating correction data before photographing a diagnosis image.
2. Description of the Related Art
Generally, X ray CT apparatuses calculate (reconstructs) an X ray absorptance index of tissues, such as internal organs, as indexes or CT values, on the basis of an X ray absorption amount by which X rays are absorbed in a subject, and obtain images (tomographic images). These CT values are generally based on the X ray absorptance of water or air, and are corrected by using correction data so as to correspond to the reference values. Examples of a method of acquiring correction data include a method of acquiring correction data by projecting images again at a plurality of projection angles data from an output image (for example, JP-A-2002-191591), a method of creating a thomographic image from data that is collected by a spiral data collecting unit (for example, JP-A-10-192272), and a method of acquiring data obtained by performing adding and averaging processes on data obtained through rotation of n times as correction data (for example, JP-A-10-248301).
However, the following problems exist in the methods of acquiring correction data according to the related art.
That is, according to the method of acquiring the correction data in the related art, with development of multi-column X-ray detectors (for example, increase in the number of detecting columns), a size of a phantom also needs to be increased. However, it is difficult to set a phantom whose size is increased in a column direction (bed longitudinal direction) to a bed, which causes a large amount of working loads. Further, in order to hold a plurality of phantoms whose diameters are different according to the number of detected columns, a large amount of storage space is required, which is not practical.
Further, according to the methods in the related art, X ray absorptance is acquired by scanning a phantom filled with water. Further, in a state where a photographic subject, such as the phantom or the like, is not disposed, scanning is carried out to obtain X ray absorptance of air. Accordingly, in order to obtain the X ray absorptance of the water or the air, it is necessary to perform a scanning operation twice in a state where the phantom is not disposed and a state where the phantom is disposed, which causes a large amount of working loads.